Description: This study was sponsored by the Society of Actuaries Health Section. This paper explores the relative significance of aging as a determinant of financial cost of health care beyond age 50, with particular attention to the effect for ages 65 and older. The paper presents concepts of aging factors and aging curves, which define relative values between ages for utilization or cost of health care services. General conclusions are drawn from Medicare data that utilization and cost differ by age, but that aging factors vary across services and may be less significant at very old ages. The author then turns to the question of measuring the significance of an aging curve assumption and what accuracy is lost in the simpler alternative of a single value across an age range.